The invention is in the field of porous materials having large surface areas for use in chemical catalysis, molecular separations, and the like.
The mechanical strength of articles that have a porous structure depend, among a variety of factors, upon the nature of the structural material, the sizes of the pores, the xe2x80x9cporosityxe2x80x9d or void volume, and the nature or shape of the pores. As a general but by no means rigorous rule, for a given rigid or brittle material, mechanical strength decreases as porosity increases. The specific surface of a porous material increases with decreasing pore size for a constant void volume, and increases also with increasing void volume. The decrease of mechanical strength with increasing porosity appears to be intuitive in that the greater the porosity, the thinner must be the walls separating adjacent pores, and hence the weaker the walls must be.
A porous material having a substantial void volume is described in Kistler, U.S. Pat. No. 2,093,454. Here, a liquid is removed from a gel at a temperature above the critical temperature of the liquid to leave a dry porous gel structure. The gel is said to undergo little shrinkage as the solvent is removed. Kistler refers to his materials as xe2x80x9caerogelsxe2x80x9d, and it is taught that these gels can contain as little as one percent or less of solid material by volume. They are, as a result, often very fragile and hence difficult to use.
Another porous material with extremely small pores is taught in xe2x80x9cUltrastable Mesostructured Silica Vesiclesxe2x80x9d, S. S. Kim, W. Zhang, T. J. Pinnavaia, Science 282, 1302-1305 (1998). This porous material is of silica, and the pores may have mean diameters ranging from about 2.7 to about 4.0 nm. The authors refer to the use of such materials in chemical catalysis and molecular separations, and the authors report having incorporated redox-active Ti(IV) and acidic Al(III) centers into the framework of their materials. Specific surfaces measured in the hundreds of m2/g are reported.
Extremely weak and fragile porous materials having pores that are to be contacted with a liquid or gas are difficult to work with because they are difficult to shape and difficult to support. They may be supported as a layer upon a supporting surface, for example. Although microporous materials themselves may have very high specific surfaces (that is, surface per unit mass), the materials themselves, when coated upon a support, provide only a limited macrosurface to come into contact with gases or liquids. Because the highly porous materials are so fragile, they cannot generally be formed into separate shapes that have greater surface areas, on a macro level, for contact with liquids or gases.
It would be desirable to provide a structure in which porous materials may be so fashioned as to render them highly available for contact to liquids and gases.
It has been found that porous materials having small pores, that is, not greater than about 100 nm and preferably in the range of about 5 to about 100 nm may themselves be supportively nested in the pores and supported on the pore walls of a relatively strong porous carrier having pore sizes at least one and preferably at least two orders of magnitude greater than the pore size of the smaller pore material. The pores of the small pore material open onto the pores of the porous carrier. By utilizing the pore walls of the porous carrier to support the smaller pore material, the available xe2x80x9cmacroxe2x80x9d surface of the smaller pore material is greatly increased so that liquids or gases flowing through the volume of pores of the porous carrier may readily access the pores of the smaller pore material. Because the area of contact between the smaller pore material and the supporting pore walls of the porous carrier material is quite large, the structural support provided by the porous carrier to the smaller pore material is substantial.
Accordingly, the invention relates to a strong, porous article comprising a porous carrier having an outer surface defining a shape having a bulk volume and having interconnecting pores providing fluid flow openings extending throughout the bulk volume and opening through the outer surface. The porous carrier has a plurality of continuous strong supportive struts defining walls bounding the pores, the pores preferably having a mean size not greater than about 100 microns. A second porous material, the mean pore size of which is at least an order of magnitude less than the mean pore size of the porous carrier, is nested within and structurally supported by the pore walls of the porous carrier so that the second porous material maintains its porous configuration. The pores of the second porous material open onto the fluid flow openings of the porous carrier and are accessible to a fluid flowing through the pores of the carrier.
In a preferred embodiment, the porous carrier has an mean pore size in the range of about 0.3 to about 10 microns, and the second porous material has a pore size at least an order of magnitude smaller, preferably in the range of about 1 to about 100 nm.
For use in describing the invention, xe2x80x9cmesoporousxe2x80x9d may be used to describe a porous material having a substantial specific surface and to which access is desired. Mesoporous materials commonly have pore sizes in the nanometer range, often ranging from about 1 to about 100 nm. These materials often are characterized by having extremely weak physical structures that often are only barely self-supporting, although some mesoporous materials can be somewhat stronger. xe2x80x9cMicroporousxe2x80x9d may be used herein to refer to pore structures having pore sizes in the micron range, particularly in the range of about 0.3 to about 10 microns. These materials commonly are much stronger in mechanical strength than the mesoporous materials. Finally, xe2x80x9cMacroporousxe2x80x9d may be used to refer to pores that are generally measured in hundreds of microns, commonly in the 100 to 500 micron range. Porous materials of this type are shown in International Publication No. WO 99/16479.
The present invention is particularly valuable in supporting mesoporous structures within the pores of a micro or macroporous structure, but on a broad basis, the invention is applicable to any combination of two porous structures in which one structure, having larger pores, serves as a support for the material having smaller pores. The pore size of the smaller pores is at least one and preferably at least two orders of magnitude less than the pore size of the carrier material. The pores of the smaller pore material open onto and communicate with the passages defined by the pores of the porous carrier. The pores of the porous carrier serve to support the second porous material, which is particularly valuable when the second porous material has little mechanical strength, especially when the second porous material is so weak as to be non self supporting or cannot be mechanically worked from one shape to another.
The sizes of pores of the various materials referred to herein are mean sizes. Conceptually, it is convenient to think of a pore having a minimum diameter of, for example, 100 microns, as being capable of accommodating the passage through it of a xe2x80x9cwormxe2x80x9d having a round cross section and a transverse diameter of 100 microns. Put another way, a 100-micron opening should enable passage through it of a sphere having a 100-micron diameter. Although I am aware of no completely satisfactory way for measuring the sizes of pores, it is possible to examine a scanning electron micrograph or other photograph of enlarged cross section of a porous material, and view the pores as planar projections of the structure. Here, lines may be drawn across the micrograph and the size of the openings that intersect the lines is measured. Averaging and standard deviation techniques may be used to permit the mean size of the openings to be assessed.
In the manufacture of articles of the invention, it is generally preferred to first complete the fabrication of the carrier porous material, that is, the material having the larger pores, and then fabricate the smaller pore material in situ. A concept central to the invention is that the material with the smaller poresxe2x80x94e.g., a mesoporous materialxe2x80x94must be situated within the pores of the larger pore material such that the smaller pores open onto the interior of the larger pores; that is, the smaller pores are accessible by a fluid flowing through the larger pores.
Preferably, the second porous material substantially fills the volume of the pores of the porous carrier so that substantially the entire surface of the pores of the second porous material is made available for contact by a liquid or gas flowing through the pores of the porous carrier. If desired, however, the second porous material may be carried by the pore walls of the carrier but may leave a central portion of the carrier pores unoccupied to facilitate the flow of a fluid through the carrier pores. This may be particularly advantageous when the surface of the second porous material is catalytic.
Although the role of the porous carrier is largely one of support, the walls of the porous carrier may be formed of, or coated with, other reactants or materials that can augment fundamental processes to be carried out by the smaller pore material. For example, while the second porous material may have one catalyst on its porous surface, the larger pore carrier material may have a second, synergistic catalyst or factor which, though present in a lesser amount, may be critical to the desired reaction. In general, the pores of the larger pore size carrier material are compatible with the smaller pore size material supported by the walls. The second porous material may be reacted with, sintered to, or adhered to the walls of the carrier material through the use of an intermediate material, for example. It will be understood that the pores of the carrier material (and for that matter preferably the pores of the second porous material) are interconnecting and extend through the volume of the material, the pores defining tortuous paths for fluid flow. If desired, the second porous material may be physically contained in the tortuously configured larger pores.
The surface of the pores of the carrier material may first be coated with a catalyst, for example, and the second porous material may be carried by the catalyst coated surface, the pores of the second porous material communicating with the catalyst bearing surface of the carrier material. As required, the carrier material itself may be formed with two or more layers. For example, the carrier material may be formed of a ceramic material such as zirconia, with thin layers of alumina, hydroxy apatite or other materials carried by the zirconia.
A variety of porous carrier materials can be employed in the invention; however, it is preferred to employ micro and macro reticulated structures that are characterized by significant mechanical strength and particularly compressive strength, and, to the extent desired, chemical inertness. Although porous materials of various types may be employed in the present invention, porous ceramics are preferred.
In the field of ceramics, in which ceramic materials are formed by sintering together ceramic particles, avoidance of porosity is the common goal to achieve maximum possible density and strength, among other desired properties. See Kingery, W. D., et al., Introduction to Ceramics, John Wiley and Sons, 1976. In some cases, however, ceramic materials that have some porosity may be useful as thermal insulation, or as materials that simulate bone. Examples of such ceramics are porous materials made from alumina, zirconia, mullite, hydroxy apatite, and mixtures or combinations of these materials. Rice, Porosity of Ceramics, Marcel Dekker, Inc., 1998, provides a review of porous ceramic materials and particularly their properties.
Porous articles may be made from powdered ceramic materials by a number of methods. For example, particles of ceramic may be simply compressed and sintered. To obtain some open pores, the particles, which may be in the form of beads, must be only slightly or partially sintered at their contact points, and this usually provides a mechanically weak structure. Further sintering improves strength, but leads to the formation of an unwanted (for the purposes of the desired products of the invention) largely closed pore structure. In another example, ceramic powders may be mixed with foaming or blowing agents; but activation of a foaming agent commonly leads to the formation of a closed pore structure. In yet another example, ceramic powders may be mixed with organic particulates which burn out when the shaped ceramic structure is raised to sintering temperatures. This procedure leads to the formation of a macroporous structure. Insulating bricks made this way, for example, may have much larger pores of this kind.
As macroporous ceramic materials, the products just described are inferior for the purposes of the invention to porous products made by coating the porous surfaces of a porous organic material such as a plastic foam or sponge with a ceramic slip, the organic material being thereafter removed as by pyrolysis, and the remaining ceramic is sintered. This method, which is a method taught in International Publication No. WO 99/16479, provides a porous product that mimics in configuration the organic material from which it was made; that is, the organic material serves as a macro xe2x80x9ctemplatexe2x80x9d for the resulting porous ceramic article. The ceramic articles made in this manner hence can have substantially completely open pores, can be carefully controlled as to pore size, and thus can serve as excellent porous carriers for products of the invention.
Another material is a microporous structure that has pores 10 to 100 fold or more smaller than the pores of the material described immediately above. This material, which suitably can be carried in the pores of a porous carrier such as the material described above, utilizes a cellulosic material as a micro xe2x80x9ctemplatexe2x80x9d to guide and structure ceramic particles. These porous articles may be made by combining cellulose or a cellulose-like polymer (that is, a gel-forming polymer) with a solvent therefore such as N-methyl morpholine oxide (hereafter sometimes referred to as xe2x80x9cNMMOxe2x80x9d) to form a sol, incorporating a powdered ceramic such as alumina in the sol, forming the sol into a desired shape, such as a strip or bead of material, extracting the solvent from the article using a non-solvent for the cellulose or other gel-forming polymer, heating to pyrolyze the cellulose or other gel forming polymer and eventually sintering the article. Here, the ceramic particles are believed to align themselves along pathways provided by the cellulosic microfibrils as the sol is reconstituted as a gel following removal of the NMMO solvent. As the resulting product is heated, water is lost and some shrinkage of the cellulose gel network occurs, the ceramic particles retaining to a significant extent their relative positions with respect to each other. Once the cellulose has been removed, as by pyrolysis, the ceramic particles, which at this point are in contact with one another, sinter together, the sintered structure reflecting at least to some extent the structure and configuration of the cellulosic template.
Yet other kinds of porous materials are mesoporous structures having pores about 100 fold smaller than those of the microporous structures described above. One such mesoporous structure is referred to in Kistler, U.S. Pat. No. 2,093,454, as an xe2x80x9caerogelxe2x80x9d. Here, water in a gel is replaced with another liquid, and that liquid is removed from the gel at a temperature above the critical temperature of the liquid to leave a dry porous gel structure. Ceramic structures made by this method do not undergo substantial shrinkage, and may have open void volumes of 95% or more but are extremely fragile.
Mesoporous materials include natural zeolite minerals or synthetic versions that have pores sized in the nanometer size range. Other mesoporous materials are those described in xe2x80x9cUltrastable Mesostructured Silica Vesiclesxe2x80x9d, S. S. Kim, W. Zhang, T. J. Pinnavaia, Science 282, 1302-1305 (1998), referred to above and incorporated herein by reference. The latter porous material is of silica, the pores having mean diameters ranging from about 2.7 to about 4.0 nm. See also F. Linsker and R. L. Evans, J. Am. Chem. Soc. 67, 1581 (1945); P. T. Tanev and T. J. Pinnavaia, Science 271, 1267 (1996); P. T. Tanev, Y. Liang and T. J. Pinnavaia, J. Am. Chem. Soc. 119, 8616 (1997), and K. M. McGrath, D. M. Dabbs, N. Yao, I. A. Aksay, S. M. Grunetz, Science 277, 552 (1997).